A high-purity diphenyl carbonate is important as a raw material for the production of an aromatic polycarbonate, which is the most widely used engineering plastics, without using toxic phosgene. As a process for producing an aromatic carbonate, a process of reacting an aromatic monohydroxy compound with phosgene has been known from long ago, and has also been the subject of a variety of studies in recent years. However, this process has the problem of using phosgene, and in addition chlorinated impurities that are difficult to separate out are present in the aromatic carbonate produced using this process, and hence this aromatic carbonate cannot be used as a raw material for the production of the aromatic polycarbonate. Because such chlorinated impurities markedly inhibit the polymerization reaction in the transesterification method which is carried out in the presence of an extremely small amount of a basic catalyst; for example, even if such chlorinated impurities are present in an amount of only 1 ppm, the polymerization hardly proceeds at all. To make the aromatic carbonate capable of using as a raw material of a transesterification method polycarbonate, a troublesome multi-stage separation/purification processes such as enough washing with a dilute aqueous alkaline solution and hot water, oil/water separation, distillation and so on are thus required. Furthermore, the yield of aromatic carbonate decreases due to hydrolysis loss and distillation loss during this separation/purification processes. Therefore, there are many problems in carrying out this method economically on an industrial scale.
On the other hand, a process for producing aromatic carbonates through transesterification reactions between a dialkyl carbonate and an aromatic monohydroxy compound is also known. However, such transesterification reactions are all equilibrium reactions. The equilibrium is biased extremely toward the original system and the reaction rate is slow, and hence there are many difficulties in producing aromatic carbonates industrially using this method. Two types of proposals have been made to improve on the above difficulties. These are developments of a catalyst to increase the reaction rate, and attempts to devise a reaction system so as to shift the equilibrium toward the product system as much as possible and thus improve the aromatic carbonate yield. For example, for the reaction between dimethyl carbonate and phenol, there have been proposed a process in which methanol produced as a by-product is distilled off by azeotropy together with an azeotrope-forming agent, a process in which the methanol produced as a by-product is removed by being adsorbed onto a molecular sieve, and a process in which, using an apparatus in which a distillation column is provided on top of a reactor, an alcohol produced as a by-product in the reaction is separated off from the reaction mixture, and at the same time unreacted starting material that evaporates is separated off by distillation (see Patent Document 1: examples in Japanese Patent Application Laid-Open No. 56-123948 (corresponding to U.S. Pat. No. 4,182,726)).
However, these reaction systems have basically been batch system or switchover system. Because there are limitations in the improvement of the reaction rate through catalyst development for such transesterification reactions, and the reaction rates are still slow, and thus it has been thought that the batch system is preferable to a continuous system. Of these, a continuous stirring tank reactor (CSTR) system in which a distillation column is provided on the top of the reactor has been proposed as the continuous system, but there are problems such as the reaction rate being slow, and a gas-liquid interface in the reactor being small, based on the volume of the liquid. Hence it is not possible to make the conversion high.
The present inventors have developed reactive distillation methods in which such a transesterification reaction is carried out in a continuous multi-stage distillation column simultaneously with separation by distillation, and have been the first in the world to disclose that such a reactive distillation system is useful for such a transesterification reaction, for example, a reactive distillation method in which a dialkyl carbonate and an aromatic hydroxy compound are continuously fed into the multi-stage distillation column, and the reaction is carried out continuously inside the column in which a catalyst is present, while continuously withdrawing a low boiling point component containing an alcohol produced as a by-product by distillation and continuously withdrawing a component containing a produced alkyl aryl carbonate from a lower portion of the column (see Patent Document 2: Japanese Patent Application Laid-Open No. 3-291257), a reactive distillation method in which an alkyl aryl carbonate is continuously fed into the multi-stage distillation column, and the reaction is carried out continuously inside the column in which a catalyst is present, while continuously withdrawing a low boiling point component containing a dialkyl carbonate produced as a by-product by distillation, and continuously withdrawing a component containing a produced diaryl carbonate from a lower portion of the column (see Patent document 3: Japanese Patent Application Laid-Open No. 4-9358), a reactive distillation method in which these reactions are carried out using two continuous multi-stage distillation columns, and hence a diaryl carbonate is produced continuously while efficiently recycling a dialkyl carbonate produced as a by-product (see Patent document 4: Japanese Patent Application Laid-Open No. 4-211038), and a reactive distillation method in which a dialkyl carbonate and an aromatic hydroxy compound or the like are continuously fed into the multi-stage distillation column, and a liquid that flows down through the column is withdrawn from a side outlet provided at an intermediate stage and/or a lowermost stage of the distillation column, and is introduced into a reactor provided outside the distillation column so as to bring about reaction, and is then introduced back through a circulating inlet provided at a stage above the stage where the outlet is provided, whereby reaction is carried out in both the reactor and the distillation column (see Patent Documents 5: Japanese Patent Application Laid-Open No. 4-224547; Patent Document 6: Japanese Patent Application Laid-Open No. 4-230242; Patent Document 7: Japanese Patent Application Laid-Open No. 4-235951).
These reactive distillation methods proposed by the present inventors are the first to enable aromatic carbonates to be produced continuously and efficiently, and many similar reactive distillation systems based on the above disclosures have been proposed thereafter (see Patent Document 8: International Publication No. 00/18720 (corresponding to U.S. Pat. No. 5,362,901); Patent Document 9: Italian Patent No. 01255746; Patent Document 10: Japanese Patent Application Laid-Open No. 6-9506 (corresponding to European Patent No. 0560159, and U.S. Pat. No. 5,282,965); Patent Document 11: Japanese Patent Application Laid-Open No. 6-41022 (corresponding to European Patent No. 0572870, and U.S. Pat. No. 5,362,901); Patent Documents 12: Japanese Patent Application Laid-Open No. 6-157424 (corresponding to European Patent No. 0582931, and U.S. Pat. No. 5,334,742); Patent Document 13: Japanese Patent Application Laid-Open No. 6-184058 (corresponding to European Patent No. 0582930, and U.S. Pat. No. 5,344,954); Patent Document 14: Japanese Patent Application Laid-Open No. 7-304713; Patent Document 15: Japanese Patent Application Laid-Open No. 9-40616; Patent Document 16: Japanese Patent Application Laid-Open No. 9-59225; Patent Document 17: Japanese Patent Application Laid-Open No. 9-110805; Patent Document 18: Japanese Patent Application Laid-Open No. 9-165357; Patent Document 19: Japanese Patent Application Laid-Open No. 9-173819; Patent Document 20: Japanese Patent Application Laid-Open No. 9-176094; Patent Document 21: Japanese Patent Application Laid-Open No. 2000-191596; Patent Document 22: Japanese Patent Application Laid-Open No. 2000-191597; Patent Document 23: Japanese Patent Application Laid-Open No. 9-194436 (corresponding to European Patent No. 0785184, and U.S. Pat. No. 5,705,673); Patent Document 24: International Publication No. 00/18720 (corresponding to U.S. Patent No. 6,093,842); Patent Document 25: International Publication No. 01/042187 (corresponding to Published Japanese Translation of PCT Application No. 2003-516376); Patent Document 26: Japanese Patent Application Laid-Open No. 2001-64234; Patent Document 27: Japanese Patent Application Laid-Open No. 2001-64235; Patent Document 28: International Publication No. 02/40439 (corresponding to U.S. Pat. Nos. 6,596,894, 6,596,895, and 6,600,061)).
Among the reactive distillation systems, the present applicants have further proposed, as a method that enables highly pure aromatic carbonates to be produced stably for a prolonged period of time without a large amount of a catalyst being required, a method in which a high boiling point material containing a catalyst component is reacted with an active substance and then separated off, and the catalyst component is recycled (see Patent Document 29: International Publication No. 97/11049 (corresponding to European Patent No. 0855384, and U.S. Pat. No. 5,872,275)), and a method carried out while keeping the weight ratio of a polyhydric aromatic hydroxy compound in the reaction system to a catalyst metal at not more than 2.0 (see Patent Document 30: Japanese Patent Application Laid-Open No. 11-92429 (corresponding to European Patent No. 1016648, and U.S. Pat. No. 6,262,210)). Furthermore, the present inventors have also proposed a method in which 70 to 99% by weight of phenol produced as a by-product in a polymerization process is used as a starting material, and diphenyl carbonate can be produced by means of the reactive distillation method. This diphenyl carbonate can be used as the raw material for polymerization to produce aromatic polycarbonates (see Patent Documents 31: Japanese Patent Application Laid-Open No. 9-255772 (corresponding to European Patent No. 0892001, and U.S. Pat. No. 5,747,609)).
As methods for separating a diaryl carbonate from the reaction mixture containing the diaryl carbonate that has been produced through transesterification reaction and the like between a dialkyl carbonate and an aromatic monohydroxy compound as a starting material as described above, and then purifying the diaryl carbonate, crystallization methods, distillation methods and the like have been proposed. With regard to the distillation methods, three methods have been proposed. One is a method in which the diaryl carbonate is obtained as a column top component from a distillation column; for example, there are:
I) a method in which the reaction mixture containing the catalyst is distilled as is in a batch type distillation column, and the diphenyl carbonate is obtained as the column top component (see Example 2 of Patent Document 10);
II) a method in which the reaction mixture containing the catalyst is subjected to flash evaporation, and thus separated into a high boiling point material containing most of the catalyst and a low boiling point material, and then the low boiling point material is distilled in a distillation column for starting material recovery, and a catalyst-containing diphenyl carbonate is obtained as a column bottom material, and then this column bottom material is distilled in a purifying column, whereby the diphenyl carbonate is obtained as a column top component (see Patent Document 33: Example 1 in Japanese Patent Application Laid-open No. 4-100824; Patent Document 34: Japanese Patent Application Laid-open No. 9-169704); and
III) a method in which the reaction mixture containing the catalyst is distilled in a distillation column (or evaporator), and thus separated into a high boiling point material containing most of the catalyst and a low boiling point material, and then the low boiling point material is subjected to continuous sequential distillation using a distillation apparatus comprising three columns, i.e. a light fraction separating column, a methyl phenyl carbonate separating column, and a diphenyl carbonate separating column, whereby diphenyl carbonate is obtained as a column top component (see Patent Document 17).
Another is a method in which the diaryl carbonate is obtained as a column bottom component from a distillation column; for example, there is:
IV) a method in which the reaction mixture containing the catalyst is distilled in a distillation column, and thus separated into a high boiling point material containing most of the catalyst and a low boiling point material, and then the low boiling point material is distilled in a distillation column, and the diphenyl carbonate is obtained as a column bottom component (see Patent Document 26).
The other is a method in which the diaryl carbonate is obtained as a side cut component from a distillation column; for example, there are:
V) a method in which the reaction mixture containing the catalyst is introduced into a third reactive distillation column, and further reaction and distillation are carried out, whereby the diphenyl carbonate is obtained as a side cut component from the reactive distillation column (see Patent Documents 12 and 13);
VI) a method in which the reaction mixture containing the catalyst is subjected to flash evaporation, and thus separated into a high boiling point material containing most of the catalyst and a low boiling point material, and then the low boiling point material is introduced into a distillation column and distillation is carried out, whereby the diphenyl carbonate is obtained as a side cut component from the reactive distillation column (see Patent Documents 30 and 31; Patent Document 35: International Publication No. 92/18458 (corresponding to U.S. Pat. No. 5,426,207);
VII) a method in which the reaction mixture containing the catalyst is distilled in a first purifying column, and thus separated into a high boiling point material containing most of the catalyst and a low boiling point material, and then the low boiling point material is introduced into a second purifying column and distillation is carried out, whereby the diphenyl carbonate is obtained as a side cut component from the second purifying column (see Patent Document 36: Japanese Patent Application Laid-open No. 11-49727); and
VIII) a method in which diphenyl carbonate containing phenyl salicylate is introduced into a distillation column having the number of theoretical stages being from 5 to 15, and distillation is carried out at a column bottom temperature of not less than 150° C., whereby the diphenyl carbonate is obtained as a side cut component from the distillation column (see Patent Document 32: Japanese Patent Application Laid-open No. 9-194437 (corresponding to European Patent No. 0784048)).
However, it has been shown that various problems remain with such diaryl carbonate separation/purification methods using these distillations. More specifically, the purity of the diphenyl carbonate obtained through the above I) is low, and moreover this is a batch process and hence is not suitable for mass production on an industrial scale. Regarding the above II), the method of Patent Document 33 is a batch method, and the diphenyl carbonate which was obtained through the method disclosed in Patent Document 34 contains a titanium catalyst, albeit in an amount of not more than 1 ppm, and hence is not suitable as a raw material for the production of a high-purity discolored polycarbonate. With the method of the above III), since the diphenyl carbonate is heated to a high temperature at the bottom of each of two of the distillation columns, i.e. the light fraction separating column and the methyl phenyl carbonate separating column, and is then subjected to a high temperature in the diphenyl carbonate separating column, the diphenyl carbonate is altered, bringing about a decrease in the purity and a decrease in the yield, which is undesirable. In actual fact, the diphenyl carbonate obtained in Example 1 in Patent Document 17 contains approximately 300 ppm of high boiling point by-products. The process of IV in which the diphenyl carbonate is obtained from the bottom of the column is unsuitable since the purity is low and hence a desired polycarbonate cannot be produced.
Since with the process of V, a reaction mixture containing all of the catalyst, unreacted starting material and impurities from the bottom of the second reactive distillation column is introduced into the third reactive distillation column from an upper portion thereof and the diphenyl carbonate is withdrawn from the side of the third reactive distillation column, vapor or mist of the starting material, the impurities, the catalyst or the like may thus be entrained, and hence the purity of the diphenyl carbonate obtained is low. The processes of VI and VII are preferable processes, but there is no mention of the presence of intermediate boiling point impurities having a boiling point between the alkyl aryl carbonate and the diaryl carbonate. Moreover, with the process of VIII, although it is stated that the content of phenyl salicylate is reduced from 3000 ppm to 50 ppm (Example 2 of Patent Document 32), nothing is mentioned whatsoever for other impurities. For example, even though the diphenyl carbonate is produced using the phosgene method in this example, and hence this is definitely a purification process for diphenyl carbonate containing chlorinated impurities, nothing is mentioned whatsoever with regard to the chlorinated impurities (which have an adverse effect on the polymerization to produce a polycarbonate and the properties of the polycarbonate even in an extremely small amount of only a few tens of ppb). With this process, such chlorinated impurities are not separated out sufficiently, and hence it is not be possible to use the diphenyl carbonate as a raw material for a polycarbonate. This is obvious from the fact that the chlorine content is 30 ppb in the diphenyl carbonate (in which after washing with alkaline hot water, and with hot water, and then water and the lower boiling point material is removed by distillation, the resulting diphenyl carbonate not containing water is purified by distillation) obtained in Comparative Example 1 of Patent Document 37 (Japanese Patent Application Laid-open No. 11-12230: this application was filed more than one year after Patent Document 32), which discloses the similar purifying method as the above process.
Furthermore, in Patent Document 32, the temperature and time at which phenol starts to be distilled off in the case that reaction is carried out with bisphenol A are given as a method of evaluating the purity of the diphenyl carbonate obtained through the distillation, but this test method cannot evaluate whether the diphenyl carbonate is suitable for polymerization. This is because even for diphenyl carbonate of low purity such that a polycarbonate of the required degree of polymerization cannot be produced, the initial reaction in which phenol is eliminated occurs sufficiently. Moreover, since with this evaluation method, a large amount of 2.3 ppm of NaOH based on the bisphenol A is used as a catalyst, even for diphenyl carbonate containing, for example, 1 ppm of chlorinated impurities, an incorrect evaluation that the diphenyl carbonate is of high purity and is suitable as a raw material for a polycarbonate would be obtained. As stated earlier, the diphenyl carbonate containing 1 ppm of chlorinated impurities cannot be used as the raw material for the polycarbonate at all. In ordinary polymerization, since such a large amount of an alkaline catalyst is not used, this evaluation method is not suitable for evaluating the purity of diphenyl carbonate to be used for producing polycarbonate. Further, in Patent Document 32, there is no specific description whatsoever of purification of diphenyl carbonate that has been obtained using the transesterification method. Since the types and contents of impurities differ between diphenyl carbonate obtained through the phosgene method and diphenyl carbonate obtained using the transesterification method, it cannot be said that diphenyl carbonate of the same purity will be obtained through the same purification method. It thus cannot be said at all that diphenyl carbonate having the required purity for the raw material of the polycarbonate would be obtained through the purification method of Patent Document 32.
A reaction mixture obtained as a column bottom component by taking as a starting material a reaction mixture containing an alkyl aryl carbonate obtained through a transesterification reaction between a dialkyl carbonate and an aromatic monohydroxy compound, continuously feeding this starting material into a reactive distillation column comprising a continuous multi-stage distillation column in which a homogeneous catalyst is present, and carrying out a transesterification reaction and distillation simultaneously in the column generally contains small amounts of various reaction by-products in addition to the diaryl carbonate, the starting material and the catalyst. Such by-products are known to include by-products having a lower boiling point than that of the aromatic monohydroxy compound used as a starting material such as an alkyl aryl ether (e.g. anisole), and by-products having a higher boiling point than that of the diaryl carbonate such as an aryloxycarbonyl-(hydroxy)-arene (e.g. phenyl salicylate) and an aryloxycarbonyl-(aryloxycarboxyl)-arene, and processes for separating these out have been proposed. For example, processes for separating out anisole (see Patent Documents 16, 17 and 20), and processes for separating out phenyl salicylate (see Patent Documents 32 and 36) have been proposed.
By carrying out more detailed studies on processes for the continuous production of the diary carbonate, the present inventors have discovered that in addition to these publicly known impurities, intermediate boiling point by-products having a boiling point between the alkyl aryl carbonate and the diaryl carbonate are also present. Hitherto, there have been no documents whatsoever disclosing the presence of such intermediate boiling point by-products or a process for removing the same. When a diaryl carbonate for which the amounts of such intermediate boiling point by-products and high boiling point by-products have not been reduced down to a sufficient level is used as the raw material of a transesterification method polycarbonate, it has been discovered that these intermediate boiling point by-products and high boiling point by-products cause discoloration and a deterioration in the properties of the polycarbonate produced. It is thus necessary to reduce the amounts of both intermediate boiling point by-products and high boiling point by-products as much as possible.